From the Royal Observatory Brussels

1) Project: Numerical study of the chemo-dynamical evolution of galaxies

       This project is articulated around two main research orientations: (1) the chemo-dynamic evolution of galaxies and (2) the modelling of the stellar thermonuclear combustion associated with explosive astrophysical phenomena (thermonuclear supernovae, thermonuclear burning at the surface of accreting compact stars).

       The axis "modelling the chemical and dynamic evolution of galaxies", started by H. Boffin at the ROB, aims at coupling the dynamic and chemical properties (stellar nucleosynthesis) of the gas to the stellar component of a galaxy. In practice, we develop a Nbody + hydrocode based on the Lagrangian TREE-SPH method. After a first phase (November 2002 - April 2003) of examining the possibilities offered by the SPH code of Boffin, it was decided to adopt the code "GADGET" (GAlaxy with Dark matter and Gas IntEracT) available from the authors at the Max-Planck Institut für Astrophysik in Garching-bei-München. The analysis of this code was undertaken in order to integrate the cooling of the gas as well as a star formation algorithm. The first application is the modelling of a prototypical spiral galaxy (the Milky Way) and, to that purpose, we have undertaken the simulation of a series of numerical test cases presented in the paper of Carraro et al (1998, MNRAS 297, 1021). This project relies on collaborations with N. Prantzos (IAP, Paris) and S. Boissier (Carnegie Obs. Pasadena, USA) and with A. El Messoudi (PhD student of M. Arnould at ULB).

       The axis "modelling of the stellar thermonuclear combustion" of the project concerns the study of the ignition and propagation phases of an explosive thermonuclear combustion in degenerate and relativistic stellar plasma. The ultimate goal of this research is the study of the nucleosynthesis associated with various cataclysmic stellar events (SNIa, Classical Novae). A concomitant objective is the development of numerical methods that can contribute in the future to the development of multidimensional (3D) stellar hydrodynamics. This research orientation is primarily multidisciplinary, as emphasized by the collaborations. Experts in terrestrial combustions will collaborate with astrophysicists on the study of astrophysical situations. In this context, links have been successfully established with non-astrophysical scientific communities, international as well as national: Service d'Astrophysique du Commissariat à l'Energie Atomique (CEA), Paris ; Laboratoire de Combustion et de Détonique, Poitiers ; Laboratoire de Mathématique Appliquée , Université de Bordeaux ; Unité de Thermodynamique de la Faculté Polytechnique, Université Catholique de Louvain). On the scientific part, Dr. Busegnies devoted the period 2002-2003 to the development of (a) an equation of state appropriate to the exploding stellar plasma, (b) an adequate Riemann Solver for this equation of state. This activity falls in line with the prospective study of a cellular thermonuclear detonation likely to develop at the surface, or in the internal part, of a white dwarf star.

Publications from the ORB/KSB
"SPH simulations of reactive shock tubes in an astrophysical perspective".
Busegnies Y, Francois J., Paulus G.
Submitted to Journal of Computational Physics, July 2003; revised version: December 2003

2) Project: Fundamental stellar parameters from binaries and young stellar groups

       Binaries are an important source of precise fundamental stellar parameters and hence provide empirical constraints on stellar evolution. In stellar groups, they provide anchor points for the interpretation of the whole stellar population. Our investigation concentrates on young stellar groups and well-detached binaries, but not exclusively since binaries in fast phases of stellar evolution and specific single stars have also a considerable potential in stellar evolution tests. In particular, the technique of spectral disentangling, which reconstructs the individual component spectra, is applied to double-lined (eclipsing) binaries in order to obtain fundamental stellar parameters of high accuracy. The stars and systems under study include:

RV Crateris, an eclipsing pre-main sequence triple, of which 59 high-resolution spectra over the whole orbital cycle of the close binary were obtained in 2003 in a cooperation with colleagues from UFMG, Belo Horizonte. Preliminary results include the radial velocity (RV) curves for the close binary and the spectral-type identification of the components (F, G and K) from disentangled parts of spectra.
Muscae, an eclipsing SB2 in Sco-Cen accompanied in a wider orbit by a less massive pre-main sequence component and further including a chemical peculiar (CP), magnetic common proper motion star. The orbital period of the eclipsing pair consisting of two B-type stars was shown to be 57.5 h, rather than the 20 d value claimed in the literature, the two eclipses were for the first time scanned in detail with uvby photometry at the SAT, and high-resolution spectra were obtained in Chile and South Africa.
HD123335, also an eclipsing SB2 in Sco-Cen, was found to be in a very eccentric orbit of 35 d, from 100 nights of uvby photometry at the SAT in 2003, supplemented by unpublished photometry from Balona at SAAO, and high-resolution spectroscopy from Chile, South Africa and New Zealand in 2002-2003. The secondary turned out to be a very slowly rotating He-weak SrTi B-type magnetic star with a rotation period of 2 months. Observing time has been obtained to exploit the unique situation that the surface of this peculiar star can be scanned during eclipse.
NGC224-#334 is a very young (single) B-type CP star in the Rosette Nebula, which we detected a few years ago. In collaboration with the group of Landstreet (Univ. Western Ontario) this 13th magnitude star was shown to possess a huge magnetic field and pronounced peculiar abundances, using FORS and UVES observations. Being the youngest known star of its class, it puts a strong empirical constraint to the time scale needed to develop the overabundances, which may turn out to be incompatible with the (slow) process of diffusion in the stellar atmosphere.
CPD-59°2628, an eclipsing, fast-rotating, massive (O9.5+B0) SB2 in the young open cluster Tr 16, was first analysed and results were published in 2001. Since then, additional spectroscopy was obtained and now cooperation with Keele Univ. (UK) will extend the data set further, making it suitable for an application of the spectral disentangling technique.
GL Carinae, another fast-rotating early-B SB2 in an eccentric orbit, shows fast apsidal motion. The collaborators in Copenhagen (CUAO) have separately made a full photometric analysis. Their spectra are currently reduced with our improved FEROS software and prepared for a spectral disentangling analysis.
AC Velorum, a hierarchical triple with an eclipsing close binary consisting of two similar, evolved, B-type stars of 6.9 and 7.1 M, has the potential to constrain the occurrence of mixing of processed material into the stellar atmosphere. The study of the light-time effect in the photometry revealed that the fainter third object consists itself of at least two stars with a combined mass of more than 6 M. The first spectroscopic evidence for the third object was found as an extra contribution in the Balmer lines. Out of the Balmer lines, the spectra of the components of the close binary have been disentangled and are used to derive atmospheric parameters and abundances.
HD 73256, a late-type star, was found by the CORALIE planet-search programme to be orbited by a 1.85 Jupiter-mass planet. A search for a planet transit with SAT in February 2003 gave a negative result, but did instead reveal the rotation period of the star.

       In several of these analyses and observational efforts, two IAP partners are involved and collaborators in Antwerp, Copenhagen, Zagreb or Belo Horizonte. In particular the important contribution of the extensive photometry at the SAT would not have been possible without the coordinated financial and scientific effort of the two IAP partners in Brussels.

       Finally, the spectra of the recurrent nova IM Normae which were obtained with FEROS in 2002 contributed to show (Duerbeck, Sterken, Baptista et al. 2003) that IM Nor is a member of the class of hybrid novae.

Publications in 2003 from the ORB/KSB and VUB
Spectroscopic and photometric observations of the recurrent nova IM Nor
Duerbeck, H.W., Sterken, C., Baptista, R., Cunha, K., Diaz, M.P., Dutra, C.M., Freyhammer, L.M., Hensberge, H., Jones, A.F.
in "Interplay of Periodic, Cyclic and Stochastic Variability in Selected Areas of the H-R Diagram" (eds. Christiaan Sterken), ASP Conf. Ser. 292, 309.
Muscae, a multiple system with a PMS component
Hensberge, H., Nitschelm, C., Bouzid, M.Y., Clausen, J.V., David, M., Freyhammer, L.M., Helt, B.E., Olsen, E.H., Sterken, C., Vaz, L.P.R.
in: "Open issues in local star formation" (eds. Jacques Lépine & Jane Gregorio-Hetem), ASSL 299, CD-Rom (ISBN 1-4020-1755-3)

Publications in 2003 from the ORB/KSB
The CORALIE survey for southern extra-solar planets. X. A Hot Jupiter orbiting HD 73256
Udry, S., Mayor, M., Clausen, J.V., Freyhammer, L.M., Helt, B.E., Lovis, C., Naef, D., Olsen, E.H., Pepe, F., Queloz, D. & Santos, N.C.
A&A 407, 679

From K.U. Leuven

1) Research carried out by Evelien Vanhollebeke

       Evelien Vanhollebeke's Ph.D. thesis is on the subject of ``The stellar population of the inner Galaxy'' and started October 1st, 2003.

       This will be in part a theoretical study and partly an observational one. The theoretical part will involve the use of a ``galactic model'' to predict number and colours of stars along a few low extinction lines of sight towards the Galactic Bulge. The confrontation with observational data, in particular the near-infrared DENIS and 2MASS surveys, the mid-infrared ISOGAL survey, and the OGLE I-band data, which also provides information on the variable nature of the sources, will allow to constrain the main characteristics of the Bulge: orientation, extent, number density, and star formation. After the initial calibration of this model, it can be used to predict numbers and arbitrary colours along any Bulge line of sight.

       The observational part of her thesis involves the metallicity of AGB stars in the bulge as derived from near-infrared spectra, and analysis of the dust content of Bulge AGB stars from mid-infrared spectra that will be obtained from a recently submitted SPITZER proposal.

       In the period October - December 2003 Evelien has reduced SOFI H,K band spectroscopy of a sample of Galactic Centre OH/IR stars. Kinematical studies had shown that this sample consisted of groups of stars with different formation histories. It is expected that the stars of the different groups have different metallicities. It is the aim of this program to prove this observationally. The data is reduced and the analysis of the spectra is ongoing in collaboration with people of the Observatoire de Besancon, Strasbourg and the Institute for Astronomy in Vienna.

       In January and February 2004, Evelien has been involved in the preparation of a proposal for the SPITZER infrared satellite. The aim of this project is to study the dust content of a group of bulge AGB stars of 1Msun. We asked for low-resolution IRS spectroscopy in the wavelength range of 5.2 - 38 microns.

2) Research carried out by Greet Decin, K.U.Leuven

       Study of the occurrence and time evolution of circumstellar debris disks and their relation with planetary systems.

       During 2003 Greet Decin has finished her PhD thesis, which was successfully defended on November 12, 2003. In the last phases of this work, she has elaborated on her earlier results and finalized an important amount of new developments, all related to the study of debris disks around normal stars.

       During 2003 it has been found that a planet is present around one of the four G dwarfs for which she detected and studied the infrared signature of a debris disk in 2000. This is the second known system where a detectable debris disk and a planet coexist.

       The detection by Greet Decin of a similar fraction of disk debris stars among G dwarfs as among A stars is in clear contradiction with studies which claim to show a decrease of the Vega phenomenon with stellar age. Consequently, Greet Decin has carried out a broad and careful study of this age dependence. It appears that several studies in the literature are flawed for two reasons: first, the found decrease of the phenomenon with age heavily depends on the inclusion in these studies of young clusters, where the circumstellar disk represents the remnant of the star formation process rather than a reprocessed disk which a debris disk such as found around Vega is in reality; second, a careful analysis of the age determinations and the true excesses reveals that for several objects the literature results are questionable. The resulting picture is one in which debris disks occur throughout the full age range, a consistent maximal fractional luminosity (IR over stellar) of the order of 0.1% is found, and a decrease is found with age for the lowest observed excesses at a given age. Presently it is not clear whether the latter effect represents a true evolutionary effect or is a statistical fluke related with the low occurrence of young stars in the solar neighbourhood.

       In collaboration with Dr. Carsten Dominik (University of Amsterdam) Greet Decin has carried out a theoretical study of the time dependence of the Vega phenomenon, considering the various known mechanisms which lead to the formation and decay of debris disks. It was found that the lifetime of a debris disk does not depend critically on its mass, but rather on the collision rate and the Poynting-Robertson effect. The large fraction of G dwarfs with debris disks can only partially be explained by the lower radiation pressure of these cooler stars.

       In this same context, the fairly high claimed occurrence of debris disks around K giants is puzzling. From a careful analysis of such objects with IRAS and ISOPHOT data it was found that in all stars except one the observed IR excesses are extended. This means that the excesses are not genuine, but due to the occurrence of the star in the vicinity of an interstellar cirrus cloud. For the remaining object the debris disk appears to be real.

       An abundance analysis has been carried out of the photospheres of stars with debris disks. Where for stars harbouring a planet the typical metallicity is larger than average, this is not the case for debris disk stars. While both distributions overlap, allowing for the existence of objects showing both phenomena, this result suggests that debris disk stars follow a regime which is genuinely different from that of stars with planetary systems.

Publications in 2003 from the KU Leuven
ISO-SWS calibration and the accurate modelling of cool-star atmospheres. IV. G9 to M2 stars
Decin, L., Vandenbussche, B., Waelkens, C., Decin, G., Eriksson, K., Gustafsson, B., Plez, B., Sauval, A. J.
Astronomy and Astrophysics, 400, 709-727
Age Dependence of the Vega Phenomenon: Theory
Dominik, C., Decin, G.
Astrophysical Journal, 598, 626-635
Age Dependence of the Vega Phenomenon: Observations
Decin, G., Dominik, C., Waters, L. B. F. M., Waelkens, C.
Astrophysical Journal, 598, 636-644

Web pages:Sandrine Sohy
Last update: June 7, 2004